"Emotions come and go. We feel euphoria, but we feel anxiety, too," said Lucy Brown of New York's Albert Einstein College of Medicine.
"This core system that is driving the person who is in love toward their sweetheart, that is much more important in a sense than an emotion," Brown told CNN.
"Romantic love is not only an emotion, it's a basic mating drive, and it's stronger than the sex drive," said anthropologist Helen Fisher, author of "Why We Love."
"It became apparent to me that romantic love was a drive -- a drive as strong as thirst, as hunger," the researcher said.
Intense passion can strike at any age, Fisher said, noting she has seen 8-year-olds and 70-year-olds who are madly in love.